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ATMEGA48-20AU

Part # ATMEGA48-20AU
Description MCU 8BIT ATMEGA RISC 4KB FLASH 3.3V/5V 32TQFP - Trays
Category IC
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Technical Document


DISCLAIMER: The information provided herein is solely for informational purposes. Customers must be aware of the suitability of this product for their application, and consider that variable factors such as Manufacturer, Product Category, Date Codes, Pictures and Descriptions may differ from available inventory.

127
2545M–AVR–09/07
ATmega48/88/168
the maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM resolution in bits can
be calculated using the following equation:
In phase and frequency correct PWM mode the counter is incremented until the counter value
matches either the value in ICR1 (WGM13:0 = 8), or the value in OCR1A (WGM13:0 = 9). The
counter has then reached the TOP and changes the count direction. The TCNT1 value will be
equal to TOP for one timer clock cycle. The timing diagram for the phase correct and frequency
correct PWM mode is shown on Figure 15-9. The figure shows phase and frequency correct
PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1 value is in the timing dia-
gram shown as a histogram for illustrating the dual-slope operation. The diagram includes non-
inverted and inverted PWM outputs. The small horizontal line marks on the TCNT1 slopes repre-
sent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set when a
compare match occurs.
Figure 15-9. Phase and Frequency Correct PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV1) is set at the same timer clock cycle as the OCR1x
Registers are updated with the double buffer value (at BOTTOM). When either OCR1A or ICR1
is used for defining the TOP value, the OC1A or ICF1 Flag set when TCNT1 has reached TOP.
The Interrupt Flags can then be used to generate an interrupt each time the counter reaches the
TOP or BOTTOM value.
When changing the TOP value the program must ensure that the new TOP value is higher or
equal to the value of all of the Compare Registers. If the TOP value is lower than any of the
Compare Registers, a compare match will never occur between the TCNT1 and the OCR1x.
As Figure 15-9 shows the output generated is, in contrast to the phase correct mode, symmetri-
cal in all periods. Since the OCR1x Registers are updated at BOTTOM, the length of the rising
and the falling slopes will always be equal. This gives symmetrical output pulses and is therefore
frequency correct.
R
PFCPWM
TOP 1+()log
2()log
-------------------------------
----
=
OCRnx/TOP Updateand
TOVn Interrupt Flag Set
(Interrupt on Bottom)
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Se
t
(Interrupt on TOP)
1 2 3 4
T
CNTn
P
eriod
O
Cnx
O
Cnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
128
2545M–AVR–09/07
ATmega48/88/168
Using the ICR1 Register for defining TOP works well when using fixed TOP values. By using
ICR1, the OCR1A Register is free to be used for generating a PWM output on OC1A. However,
if the base PWM frequency is actively changed by changing the TOP value, using the OCR1A as
TOP is clearly a better choice due to its double buffer feature.
In phase and frequency correct PWM mode, the compare units allow generation of PWM wave-
forms on the OC1x pins. Setting the COM1x1:0 bits to two will produce a non-inverted PWM and
an inverted PWM output can be generated by setting the COM1x1:0 to three (See Table on
page 132). The actual OC1x value will only be visible on the port pin if the data direction for the
port pin is set as output (DDR_OC1x). The PWM waveform is generated by setting (or clearing)
the OC1x Register at the compare match between OCR1x and TCNT1 when the counter incre-
ments, and clearing (or setting) the OC1x Register at compare match between OCR1x and
TCNT1 when the counter decrements. The PWM frequency for the output when using phase
and frequency correct PWM can be calculated by the following equation:
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x Register represents special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR1x is set equal to BOTTOM the
output will be continuously low and if set equal to TOP the output will be set to high for non-
inverted PWM mode. For inverted PWM the output will have the opposite logic values. If OCR1A
is used to define the TOP value (WGM13:0 = 9) and COM1A1:0 = 1, the OC1A output will toggle
with a 50% duty cycle.
15.10 Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clk
T1
) is therefore shown as a
clock enable signal in the following figures. The figures include information on when Interrupt
Flags are set, and when the OCR1x Register is updated with the OCR1x buffer value (only for
modes utilizing double buffering). Figure 15-10 shows a timing diagram for the setting of OCF1x.
Figure 15-10. Timer/Counter Timing Diagram, Setting of OCF1x, no Prescaling
Figure 15-11 shows the same timing data, but with the prescaler enabled.
f
OCnxPFCPWM
f
clk_I/O
2 NTOP⋅⋅
----------------------------=
clk
Tn
(
clk
I/O
/1)
O
CFnx
clk
I/O
O
CRnx
T
CNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
129
2545M–AVR–09/07
ATmega48/88/168
Figure 15-11. Timer/Counter Timing Diagram, Setting of OCF1x, with Prescaler (f
clk_I/O
/8)
Figure 15-12 shows the count sequence close to TOP in various modes. When using phase and
frequency correct PWM mode the OCR1x Register is updated at BOTTOM. The timing diagrams
will be the same, but TOP should be replaced by BOTTOM, TOP-1 by BOTTOM+1 and so on.
The same renaming applies for modes that set the TOV1 Flag at BOTTOM.
Figure 15-12. Timer/Counter Timing Diagram, no Prescaling
Figure 15-13 shows the same timing data, but with the prescaler enabled.
O
CFnx
O
CRnx
T
CNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
clk
I/O
clk
Tn
(
clk
I/O
/8)
TOVn (FPWM)
and ICFn (if used
as TOP)
OCRnx
(Update at TOP)
TCNTn
(CTC and FPWM)
TCNTn
(
PC and PFC PWM)
TOP - 1 TOP TOP - 1 TOP - 2
Old OCRnx Value
New OCRnx Value
TOP - 1 TOP BOTTOM BOTTOM + 1
clk
Tn
(clk
I/O
/1)
clk
I/O
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